The already dramatic Arizona Senate race flipped Thursday with Democrat Rep. Kyrsten Sinema taking the lead over Republican Rep. Martha McSally after the day’s vote count was added to the overall tally. Sinema started the day down 1 point or some 17,000 votes, but surged to the lead with the inclusion of votes counted from two of the state’s largest counties. The race for what was Sen. Jeff Flake’s seat is a consequential one that Sinema had led in the polls throughout much of the campaign, but tightened in the weeks before Election Day as $30 million combined was dumped in the race by both parties. Sinema’s margin now stands at less than half a point and there are still as many as 350,000 votes statewide that need to be counted. According to the Arizona Secretary of State’s website, five counties remain that have yet to report their full counts.

Of the some 350,000 uncounted votes, nearly all are in Maricopa County, according to the local ABC News affiliate, which reports that state election officials “estimate about 195,000 of those are early ballots, provisional, and out-of-precinct ballots that voters cast or dropped off on Election Day.” Maricopa County includes the city of Phoenix as well as Sinema’s congressional district. Thursday vote total expanded what had been a slight 1 percent edge for Sinema in the county to 2.5 points.

The count seems certain to spill into next week as four county Republican parties have filed suit saying that the methodology for allowing voters to adjust for problems with their mail-in votes isn’t uniform. Lawyers for Maricopa County said in a hearing Thursday the discrepancy affects a small percentage of the county’s nearly 500,000 votes. Part of what appears to be slowing the entire counting process down is that roughly 75 percent of voters in the state cast ballots by mail, which, according to local news radio station KTAR, means “those ballots have to go through the laborious signature confirmation process, and only then can be opened and tabulated.”